In recent years, thin film devices that include organic semiconductor materials such as organic EL devices, organic FET (field-effect transistor) devices, and organic thin-film photoelectric conversion devices have attracted attention, and have started putting into practical use. Of the fundamental physical characteristics of organic semiconductor materials for use in these thin film devices, carrier mobility and an on/off ratio are important. For example, in organic EL devices, carrier mobility, which influences charge transport efficiency, is important for highly efficient light emission and driving at low voltages. Also, in organic FET devices, carrier mobility and on/off ratios, which directly influence switching speed and the performance of a device to be driven, are important for putting organic FET devices into practical use.
Also, in order to make efficient use of the characteristics of organic semiconductor materials in these thin film devices, stable driving in the atmosphere is important. Stable driving in the atmosphere would make operations under an inert atmosphere and sealing and the like unnecessary. Thus, production processes can be simplified, and costs for equipment required for production can be greatly reduced.
Conventionally, in organic semiconductor materials, as with inorganic semiconductor materials, organic semiconductor materials for use in p-type (i.e., hole-transporting) transistors (referred to as “p-type material” hereinbelow) and organic semiconductor materials for use in n-type (i.e., electron-transporting) transistors (referred to as “n-type material” hereinbelow) are known. For example, in order to fabricate logical circuits such as CMOS (complementary metal oxide semiconductors), p-type materials and n-type materials have been required.
Up to now, a lot of research on p-type materials has been conducted, and materials that have high performance and are driven stably in the atmosphere have been reported. In contrast, as for n-type materials, researches have not greatly advanced, and limited materials are stably driven in the atmosphere.
One example of the n-type materials driven stably in the atmosphere is compounds having a quinoid structure. Of these, thienoquinoid compounds have been widely investigated, and high-performance materials such as oligothiophene quinoid materials and benzodithiophene quinoid materials have been developed (Patent Literatures 1 to 2 and Non-patent Literatures 1 to 3). On the other hand, although compounds having a benzoquinoid structure may have a possibility of being an n-type material having stability in the atmosphere and high performance, very few researches have been made on FET materials having a benzoquinoid structure.
Patent Literature 3 and Non-patent Literature 4 describe an organic semiconductor material that has a structure represented by the chemical formula:
and can be used for organic FET devices. They show that the material has higher electron mobility and stability under atmospheric conditions than tetracyanoquinodimethane. However, the material exhibits high off current, thereby decreasing an on/off ratio, and thus may not constitute a practical transistor.